This invention relates generally to radio frequency (RF) excited gas plasma reaction devices and more particularly to a reaction device to low temperature ashing of organic specimens prior to trace element analysis.
In the laboratory techniques of analysis of inorganic content of organic substances, various means have been used to decompose the organic substance so that the residual inorganic content may be analyzed. Among these, the method of wet ashing is widely used to decompose the organic constituents of the sample. In the wet ashing process, relatively large quantities of hot mineral acids are used for the decomposition process. Aside from the complexity of handling the large quantities of acid and the attendant danger in conducting the process, the technique suffers the disadvantage that the acids used must be extremely pure, otherwise contaminants in the acids will reduce the accuracy of the analysis being made. Furthermore, with the wet digestion process, an operator must be present at all times during the process to deal with foaming, bumping, spattering, and so forth.
Due to the disadvantages of the wet digestion process and requirement for more accurate determination of trace elements in organics, there was a requirement for other methods of reduction of organics to produce the required data. Among these, the most promising has been the use of a plasma for low temperature reduction of organics. In this technique, a reaction gas, such for example as oxygen, is coupled to the output of an isolater operating at radio frequencies in a chamber to form a plasma which then reacts with the organic substance to reduce that substance to ash at low temperature. Chemical changes are brought about at relatively low temperatures in the organic materials by chemically active neutral species which are produced by excitation of the low pressure gas by the RF electric field. The chemically active neutral species are produced in the field through the collision of accelerated ions and electrons with neutral gas molecules. The active species are relatively susceptible to recombination or destruction, particularly by contacting the walls of the containing chamber. The rate of destruction of the active species at the walls is directly dependent upon the wall temperature and it is therefore important that the temperature of the walls of the chamber be maintained as low as is possible.
There are, in the prior art, several devices which accomplish the general objectives of low temperature ashing of organic substances through the use of an RF plasma. Among the disadvantages of the prior art devices are the requirements for an inordinately high power to accomplish the required ashing of organics with the resultant increase or inordinately high level of temperature of the reaction chamber walls. This requires that special care be taken to minimize the contact of the plasma with the chamber walls so that destruction of the active species will be kept at a minimum.
Of the prior patents and publications noted pertaining to the invention, the patent to Bersin, U.S. Pat. No. 3,410,776; the patent to Gleit, U.S. Pat. No. 3,547,802; and two publications, Chester E. Gleit and Walter D. Holland, "Use of Electrically Excited Oxygen for the Low Temperature Decomposition of Organic Substances," Analytical Chemistry, 34:1454 (1962), and Chester E. Gleit, "High Frequency Electrodeless Discharge System for Ashing Organic Matter," Analytical Chemistry, 37:314 (1965); the patent to Gorin, U.S. Pat. No. 3,775,621; and the patent to Mitzel, U.S. Pat. No. 3,875,068 all relate to RF gas plasma reduction apparatus of a type having a helical or solenoid antenna around the reduction chamber. As will be discussed in greater detail below, this type of antenna and approach to formation of the plasma is relatively inefficient and requires large input of energy to obtain the required reduction of organic substances. The prior art patents to Hollahan, U.S. Pat. No. 3,428,548, Gorin, U.S. Pat. No. 3,619,403, and Bersin, U.S. Pat. Nos. 3,647,676 and 3,671,195 relate to gas plasma devices having split antennas. This type of antenna has the disadvantage of imparting undue heating to the walls of the reduction chamber with the attendant disadvantages of destruction of active species set forth above. The patent to MacLean, U.S. Pat. No. 3,205,162 relates to a liquid cooling apparatus for an electric discharge process somewhat similar to a cooling device disclosed in the present invention. This reference is distinguished over the present invention in that the equivalent of the "antenna" for MacLean is within the cooling device with attendant disadvantages to be discussed in detail hereinafter.